1. Field of the Invention
The present invention relates to a pressure sensor, particularly to an improvement in a sealing portion of a pressure sensor for measuring a fuel pressure of mainly gasoline and the like for an internal combustion engine.
The present invention also relates to a fuel tank provided, for instance, in a fuel system of an engine for an automobile and the like, having a pressure sensor for detecting a variation in an inner pressure thereof.
2. Discussion of Background
FIG. 2 is a partially broken sectional diagram showing a sealing means of a conventional pressure sensor disclosed in Japanese Unexamined Utility Model Publication No 192234/1987. In FIG. 2, a reference numeral 1 designates a pressure sensing element wherein resistors are formed on a thin film silicon chip in a bridge-like form and a pressure is detected by employing the piezoresistance effect wherein the resistance balance varies by receiving the pressure, and reference numeral Z designates a substrate for mounting the pressure sensing element 1 wherein four leads (not shown) are soldered which are connected to a hybrid integrated circuit (not shown) for ampliciation and compensation.
Further, in FIG. 2, a numeral 3 designates an O ring for sealing a pressurized medium to prevent leakage to the surroundings when the pressure P communicates with the pressure sensing element 1 through a nipple 5 and which is, for instance, made of a silicone species resin (fluorosilicone). Numeral 4 designates a filter for removing dust or other foreign objects in the pressurized medium. Numerals 6 and 7, designate a base and a cover for accommodating respective parts such as the pressure sensing element 1, the substrate 2, the O ring 3 and the filter 4. Numeral 8 designates a connector for receiving power supply from a computer (not shown) and for sending back an output signal in accordance with the pressure P to the computer. Numeral 9 designates a cup. Numeral 1 designates a pipe which is welded to the pressure sensing element 1 and which introduces the pressure P to the silicon chip. Numeral 11 designates the overall apparatus of the pressure sensor.
Next, an explanation will be given of the operation of the conventional pressure sensor 11. In the internal structure thereof, the cup 9 is welded to the base 6 at its peripheral portion contacting the base 6 such that the pressurized medium does not leak into the pressure sensor 11 through a clearance between the cup 9 and the base 6. The pressurized medium passes through the nipple 5 and fine dust and dirt are removed by the filter 4. The pressure P communicates with the pressure sensing element 1 through the pipe 10.
With this arrangement, the pressure P is read by the pressure sensing element 1 and the hybrid integrated circuit (HIC) accurately and swiftly without leaking the pressurized medium through a clearance between the cup 9 and the pipe 10 by the operation of the O ring 3. The read value is outputted to the computer through the connector 8. Since the sealing means of the conventional pressure sensor is composed as above, when the pressure of the liquid phase of gasoline or gasoline vapor is measured, the sealing performance of the O ring made of a silicone species material is deteriorated by permeating of gasoline vapor or by an expansion thereof due to absorbing of gasoline owing to the physical property as shown in the following Table 1. Further, as shown in Table 1, when an O ring made of a fluorine species material having gasoline resistance is employed, there is a change of shape in the O ring at a low temperature, which also amounts to the deterioration of the sealing performance.
TABLE 1 ______________________________________ Fluorine Silicone species species O ring O ring ______________________________________ Heat resistance High temperature .largecircle. .largecircle. Low temperature X .largecircle. Gasoline resistance .largecircle. X Gasoline vapor permeating .largecircle. X resistance ______________________________________
FIG. 5 is an exploded perspective view showing an example (for instance, Japanese Patent Application No. 95086/1991) of a conventional fuel tank. In FIG. 5, a reference numeral 101 designates a main body of a fuel tank accommodating fuel, which is composed of an upper casing 102 and a lower casing 103. Further, two openings 102a and 102b are provided on the upper casing 102. A numeral 104 designates two sheets of disc-like plates provided to the main body 101 of the fuel tank for closing the openings 102a and 102b, respectively, and 105, a ring-like packing for interposing between the respective plate 104 and the main body 101 of the fuel tank.
FIG. 6 is a perspective view magnifying the plate 104 in FIG. 5. The plate 104 is attached with a fuel pump 106, a fuel level measuring unit 107, a pressure sensor 108, an electric signal terminal 109, and pipes 110 and the like.
The fuel tank is completely sealed at its joint portions composing elements for preserving a flammable and volatile fuel, such as gasoline in the main body 101 of the fuel tank. The upper casing 102 and the lower casing 103 composing the main body 101 of the fuel tank are welded together and a packing 105 is provided between the plate 104 and the main body 101 of the fuel tank. Further, the pipe 110, and the fuel pump 106 and the electric signal terminal 109 connected to the fuel level measuring unit 107, are connected to the plate 104 in an airtight fashion respectively by soldering and by an adhesive agent, respectively. Further, an O ring 111 is provided at the outer periphery of a nipple 108a which is the pressure introducing portion of the pressure sensor 108, as shown in FIG. 7.
In the conventional fuel tank constructed as above, a single O ring 111 is provided as a means for sealing the pressure sensor 108. Therefore, the sealing performance of this portion is directly influenced by the material of the O ring 111. For instance, when the pressure of the liquid phase of gasoline or the pressure of gasoline vapor is measured, and when the O ring 111 is made of a silicone species gummy material, the sealing performance is deteriorated by the permeation of gasoline vapor or by expansion due to gasoline absorption. Further, when employing an O ring 111 made of a gasoline resistant fluorine species gummy material, the sealing performance thereof is also deteriorated since the of shape is changed by low temperatures. The pressure change in the main body 101 of the fuel tank is very small and a very small quantity of pressure leakage hampers the accurate detection of pressure changes. Therefore, when the sealing performance of the O ring 111 is deteriorated as above, pressure detection accuracy becomes poor and the reliability is lowered.